1. Field
The invention relates to a rubber composition based on at least one diene elastomer, a filler comprising at least carbon black and an inorganic filler, in particular silica, this composition having a very good dispersion of filler in the elastomeric matrix. The invention relates more particularly to the preparation of such a composition based on at least one masterbatch comprising the diene elastomer and the carbon black, said masterbatch itself having a very good dispersion of the carbon black in the elastomeric matrix.
The term “masterbatch” is understood to mean: an elastomer-based composite into which a filler and optionally other additives have been introduced.
The present invention relates in particular to the use of such a masterbatch for the manufacture of diene rubber compositions reinforced with a blend of organic filler and inorganic filler, which are intended for the manufacture of tires or of semi-finished products for tires, in particular treads for these tires.
2. Description of Related Art
It is known that in order to obtain the optimum reinforcing properties and hysteresis properties imparted by a filler to a tire tread, and thus to obtain high wear resistance and low rolling resistance, it is generally advisable for this filler to be present in the elastomeric matrix in a final form that is both as finely divided as possible and as uniformly distributed as possible. However, such conditions can be achieved only if this filler has a very good capacity, on the one hand, to be incorporated into the matrix during the mixing with the elastomer and to deagglomerate, and, on the other hand, to disperse uniformly in this matrix.
Since fuel savings and the need to protect the environment have become a priority, it has proved necessary to produce tires that have a reduced rolling resistance without adversely affecting their wear resistance.
This has been made possible in particular by virtue of the use, in the treads of these tires, of novel rubber compositions reinforced at least partially with inorganic fillers, in particular specific silicas of the highly dispersible type, that are capable of rivaling from the reinforcing standpoint a conventional tire-grade carbon black, while offering these compositions a lower hysteresis, which is synonymous with a lower rolling resistance for tires containing them, and also improved grip on wet, snow-covered or icy ground.
However, for reciprocal affinity reasons, these inorganic filler particles have an annoying tendency to clump together in the elastomeric matrix. These interactions have the deleterious consequence of limiting the dispersion of the filler and therefore the reinforcing properties to a level substantially below that which it would be theoretically possible to achieve if all the (inorganic filler/elastomer) bonds capable of being created during the compounding operation were actually obtained. These interactions moreover tend to increase the viscosity in the uncured state of the rubber compositions and therefore to make them more difficult to process than when carbon black is present, even for highly dispersible silicas.
There are various methods for obtaining a masterbatch of diene elastomer and reinforcing filler. In particular, one type of solution consists, in order to improve the dispersiblity of the filler in the elastomeric matrix, in compounding the elastomer and the filler in the “liquid” phase. To do so, the process involves an elastomer in latex form, which is in the form of water-dispersed elastomer particles, and an aqueous dispersion of the filler, that is to say a filler dispersed in water, commonly referred to as a “slurry”. Certain processes in particular, such as those described in document U.S. Pat. No. 6,048,923, make it possible to obtain a masterbatch of elastomer and filler that has a very good dispersion of the filler in the elastomeric matrix, greatly improved compared to the dispersion of the filler in the elastomeric matrix capable of being obtained during the solid-phase compounding of elastomer and reinforcing filler. This process consists in particular in incorporating a continuous flow of a first fluid consisting of an elastomer latex into the compounding zone of a coagulation reactor, in incorporating a second continuous flow of a second fluid consisting of an aqueous dispersion of filler under pressure into the compounding zone to form a mixture with the elastomer latex, the compounding of these two fluids being sufficiently energetic to make it possible to almost completely coagulate the elastomer latex with the filler before the outlet orifice of the coagulation reactor, and then in drying the coagulum obtained.
This process is particularly suitable for producing a masterbatch that has a very good dispersion, starting from a natural rubber latex and carbon black. Indeed, the application of this process is rendered particularly favourable by the ability that the natural rubber latex and carbon black have to coagulate together spontaneously. Conversely, silica does not coagulate spontaneously with the natural rubber latex since the silica aggregates are typically hydrophilic in nature and have greater affinity with water than with the elastomer particles themselves.
Furthermore, such a process has a limit as regards the content of carbon black present in the masterbatch, however the subsequent incorporation of carbon black in solid form, to increase the overall filler content in the elastomeric matrix, does not make it possible to retain the properties of very good dispersion of the filler in the elastomeric matrix that were obtained previously, and consequently does not make it possible to retain the advantages obtained for the hysteresis.